The present invention relates to light conductors, particularly of the type constituted by optical fibers housed in a tubular sheath.
The use of light conductors in place of or in addition to electrical conductors for transmission of information signals has been known in theory for some time and has resulted in the use of, preferably, glass fibers or glass filaments, but in any case long, thin, flexible, artificially produced structures of glass, as light conductors in addition to other solid and liquid substances. The advances in the art of low-loss glasses and quartzes has further resulted in the development of limited length light conductors for endoscopes and transmission and amplification devices, for example, as well as light conductors of effectively unlimited length, for example in the form of cables known as light cables or light conducting cables.
There exist, in addition to bare cables, sheathed light conductors in which an individual cylindrical, light transmitting, dielectric body or a bundle of such bodies is embedded in a sheathing medium having an optical index of refraction different from that of the bodies. The performance of these sheathed light conductors is aided by the total reflection which will occur at the boundary between the materials having different refractive indices for certain values of the ratio between those indices.
Furthermore, a distinction is made between single mode and plural mode light conductors.
It has also been known for a long time to enclose glass fiber light conductors individually or as a group in an outer flexible protective covering which is independent of the configuration of the conductors themselves, or to at least fasten conductors on an additional, preferably band-shaped, carrier or between two such carriers, in particular to glue them thereto. Both methods are intended to prevent mechanical damage to the glass fiber light conductors which are known to have a high tensile strength but insufficient shear resistance. Thermoplastic synthetics such as polyethylene, polyesters, polyamides or polyacrylates have been considered as sheathing materials as well as for the carrier tapes. In addition it is also known to use woven hoses, which are lacquered or resin-saturated and then hardened, for the sheathings.
In addition to tightly wrapping the outer, often multi-layer, protective sheathing around the glass fiber light conductor or conductors, it is also known to use sheathings which leave cavities around the conductors, particularly sheathings which are provided with internal, e.g. cam-like, protrusions which act as spacers or to use sheaths with a smooth interior in which one or a plurality of turns of threads, or filaments, are wound to provide a spacing between the glass fiber light conductor and the protective sheathing.
Another known cavity sheathing construction involves a continuous covering of a glass fiber light conductor with a cellular polyethylene layer extruded around the conductor and the disposition of a plurality of glass fiber light conductors between the arms of a star-profile central plastic core which is encased in an outer plastic coating, the arrangement being such that cavities are provided therein.
In all these numerous known embodiments, the glass fiber light conductors are arranged to be as straight as possible and are stretched out inside their protective sheathing and/or in their additional carriers. These arrangements are obviously based on the endeavor to avoid bends in the glass fiber light conductors of a light conducting cable as much as possible in order to reduce transmission attenuation, although it is known, on the other hand, that curvatures exceeding a certain radii of curvature of the order of magnitude of centimeters have no influence on the light conduction at the light wavelengths employed for transmission.